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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Tue, 29 Oct 2024 16:03:12 -0000 (UTC)
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john larkin <JL@gct.com> wrote:
> On Mon, 28 Oct 2024 20:19:25 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 2024-10-28 08:14, piglet wrote:
>>> john larkin <JL@gct.com> wrote:
>>>> On Sun, 27 Oct 2024 22:59:59 -0000 (UTC), piglet
>>>> <erichpwagner@hotmail.com> wrote:
>>>> 
>>>>> Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
>>>>>> On 27-10-2024 13:26, Klaus Vestergaard Kragelund wrote:
>>>>>>> On 27-10-2024 03:26, john larkin wrote:
>>>>>>>> On Sun, 27 Oct 2024 02:19:14 +0200, Klaus Vestergaard Kragelund
>>>>>>>> <klauskvik@hotmail.com> wrote:
>>>>>>>> 
>>>>>>>>> Hi
>>>>>>>>> 
>>>>>>>>> I am working on an IR detector that will guide a robot into a docking
>>>>>>>>> station.
>>>>>>>>> 
>>>>>>>>> A IR transmitter on the docking station transmits a beam, and 2 IR
>>>>>>>>> detectors on the robot detects the beam and lets the robot navigate
>>>>>>>>> towards the target. The working distance is a couple of meters.
>>>>>>>>> 
>>>>>>>>> I need it to be insensitive to ambient light/sunlight.
>>>>>>>>> 
>>>>>>>>> The IR detectors are placed in a tube, to narrow in the beam angle and
>>>>>>>>> to avoid sunlight (since it is seldom the sun is actually that low in
>>>>>>>>> the horizon)
>>>>>>>>> 
>>>>>>>>> The IR transmitter will be modulated with 10kHz (TBD) frequency, low
>>>>>>>>> duty cycle. Low duty cycle to be able to drive the LED with high
>>>>>>>>> current, frequency modulated so that the receiver can ignore the effect
>>>>>>>>> of daylight (DC)
>>>>>>>>> 
>>>>>>>>> If the LED on the docking station has higher radiant intensity at the
>>>>>>>>> point of the robot (2 meters away) than possible IR from sunlight, then
>>>>>>>>> that would be perfect.
>>>>>>>>> 
>>>>>>>>> Example of transmitter:
>>>>>>>>> 
>>>>>>>>> https://www.vishay.com/docs/83398/vsmy2850.pdf
>>>>>>>>> 
>>>>>>>>> Has up to 1000mW/sr. Seems my basic calculation for a 15 degree beam,
>>>>>>>>> shows less than 10nW/m2, while sunlight has 1W/m2. So driving a beam
>>>>>>>>> that has higher output than sunlight seems unlikely.
>>>>>>>>> 
>>>>>>>>> I would use a IR phototransistor at 850nm, something like this:
>>>>>>>>> 
>>>>>>>>> https://www.ttelectronics.com/TTElectronics/media/ProductFiles/
>>>>>>>>> Datasheet/OP505-506-535-705.pdf
>>>>>>>>> 
>>>>>>>>> Or a photo diode:
>>>>>>>>> 
>>>>>>>>> https://docs.rs-online.com/9f58/0900766b816d8a09.pdf
>>>>>>>>> 
>>>>>>>>> Fed from reverse 3.3V and into a transimpedance amplifier to boost the
>>>>>>>>> signal with bandpass filter.
>>>>>>>>> 
>>>>>>>>> One can get digital IR detector used in a remote control systems:
>>>>>>>>> 
>>>>>>>>> https://www.vishay.com/docs/82491/tsop382.pdf
>>>>>>>>> 
>>>>>>>>> It has AGC, but digital output. I need analog output to be able to zero
>>>>>>>>> in on the transmitter beam.
>>>>>>>>> 
>>>>>>>>> I have been looking for IR detectors that has the analog output, not
>>>>>>>>> just the digital, but have not found any.
>>>>>>>>> 
>>>>>>>>> If the photodiode detector is subjected to sunlight, I am guessing I
>>>>>>>>> would need very high gain on the 10kHz modulation frequency to pick up
>>>>>>>>> the burried signal in the DC from sunlight.
>>>>>>>>> 
>>>>>>>>> How do I best bias the photo diode for optimum detection of the 10kHz
>>>>>>>>> signal while being immune to the ambient sunlight?
>>>>>>>>> 
>>>>>>>>> I have chosen 850nm which seems to be a good wavelength. The spectrum at
>>>>>>>>> sea level has some dips due to water absorption.
>>>>>>>>> 
>>>>>>>>> https://sciencetech-inc.com/web/image/49169/
>>>>>>>>> Spectrum%20with_out%20absorption.png
>>>>>>>>> 
>>>>>>>>> Seems like 750nm would be better, since then the IR from the sun is
>>>>>>>>> lower, but does reduced the effective range of the system during
>>>>>>>>> fog/rain. Probably that's why these system do not use 750nm
>>>>>>>>> 
>>>>>>>>> Other considerations?
>>>>>>>> 
>>>>>>>> You could drive the LED with a square wave, 10 KHz or whatever. The
>>>>>>>> photodiode could have +DC on one end and the other end can hit a
>>>>>>>> parallel LC to ground, resonant at 10K.
>>>>>>>> 
>>>>>>>> That takes out the sunlight DC component and adds bandpass filtering.
>>>>>>>> 
>>>>>>> 
>>>>>>> That's a very nice idea. The Q should not matter much, just as long as
>>>>>>> DC is removed.
>>>>>>> 
>>>>>>> The photodiode will still be subjected to the high ambient light, but
>>>>>>> the gain would be close to zero for the stage after. I would then still
>>>>>>> need to be sure the photodiode is never saturated by ambient light.
>>>>>>> 
>>>>>> Actually, wont a simple high pass filter work equally well?
>>>>>> 
>>>>>> Photo diode with bias -> capacitor to gain block....
>>>>>> 
>>>>> 
>>>>> If you make it part of the bias network yes, a gyrator either a single
>>>>> transistor or op amp could take the role of inductor.
>>>> 
>>>> When you need an inductor, an inductor makes an excellent inductor.
>>>> 
>>>> But the dual microphone thing, electret or mems, sounds like a much
>>>> better way to go.
>>>> 
>>>> 
>>> 
>>> Some folk are scared of inductors. 10kHz does mean quite a few milli
>>> henries.
>>> 
>>> Of course they might be able to reverse the process and have one receiver
>>> on the robot and two emitters on the docking station, a bit like aircraft
>>> VOR
>> 
>> Or like certain radio navigation systems popular in Western Europe, 80 
>> or so years ago. ;)
>> 
>> That's a good example of Horace Darwin's maxim, "Always try it the other 
>> way round", quoted in "Instruments and Experiences" by R. V. Jones.
>> 
>> Jones is one of my technical heroes, and was instrumental in defeating a 
>> couple of the aforementioned navigation systems, specifically 
>> "Knickebein" and "X-Geraet", as explained in his book, "The Wizard War" 
>> (entitled "Most Secret War" in the UK).  Horace Darwin was a grandson of 
>> Charles Darwin, and a colleague of Jones's at Aberdeen after the war.
>> 
>> Cheers
>> 
>> Phil Hobbs
> 
> https://www.amazon.com/Instruments-Experiences-Measurement-Instrument-Technology/dp/047191763X
> 
> 
> 

Cheap at twice the price!

It’s a unique book—a collection of Jones’s groundbreaking papers on things
like measuring the ether drag on light in a moving dielectric, which would
be a niche interest except that each one is paired with an essay of about
equal length explaining how the measurement was done. 

Those pieces are of lasting value—they’re confidential in tone, including
the inventions required, the blind alleys, and very often, how somebody
made a big difference with a piece of hard-won advice, like that remark of
Darwin’s. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics